This invention generally relates to fuse housing assemblies for liquid-cooled transformers. More particularly, the invention relates to a vent plug positioned in a vent hole of a fuse holder housing assembly which prevents large quantities of transformer oil from being ejected, but still allows for venting of the gas buildup, when the fuse holder is removed.
A bayonet-type fuse assembly is one mechanism to protect a transformer or an electric circuit from excessive current. The fuse will serve to protect the transformer or electrical system by short circuiting and disrupting the flow of electricity upon overload or system failure. In such an assembly, the fuse is designed to operate under a dielectric oil, such as mineral oil, within the transformer. This submersion is ideal as it helps to extinguish any electrical arc between the conductive opposite ends of the fuse when the fuse operates or melts in response to excessive current.
In a conventional bayonet-type fuse assembly, a cylindrical housing of electrically insulating material protrudes into the transformer at a downward angle such that a portion of the housing is located on the interior of the transformer and a portion of the housing is located on the exterior of the housing. In order for the fuse to be submersed in the oil inside the transformer, it is necessary that at least some portion of the housing on the interior of the transformer be submersed in the oil as well.
In order for the fuse housing assembly to receive the fuse, a bayonet-type fuse holder will have an attached fuse cartridge. This fuse holder may be inserted into the interior of the cylindrical fuse housing such that upon pushing the fuse holder into the fuse housing, the fuse becomes submersed in oil in the interior portion of the transformer and is in contact with a pair of contacts. The portion of the fuse holder on the exterior of the transformer will seal the fuse housing when the holder is inserted into the housing. After the fuse has operated, the fuse holder and attached fuse may be removed from the fuse housing. The fuse may then be replaced and the holder reinserted to permit the transformer to operate.
During operation transformers can become extremely hot as a result of high load conditions, as well as high ambient temperatures. Upon an increase in the temperature of the transformer, the dielectric oil in the interior of the transformer will expand and the transformer can become highly pressurized. As the volume of the oil changes in light of the surrounding conditions, it is apparent that the level of the oil in the interior of the transformer will fluctuate as well. This fluctuation presents difficulties when removing the fuse holder as it is possible that the oil level may approach the level of the opening of the fuse housing assembly, or may even rise above it. Upon removal of the fuse holder, there is the possibility of a loss of oil because of the higher level of oil in the transformer combined with the difference in pressure between the inside of the transformer and the outside atmosphere. Because of the high temperature of the oil inside the transformer, a loss of oil, especially in large quantities, can result in a dire safety hazard for those servicing the transformer. In fact, it is quite possible that a high velocity liquid stream of hot oil may emit from the portion of the fuse housing on the exterior of the transformer.
Because it is necessary for the fuse to be in contact with the dielectric oil inside the transformer when functioning, a loss of oil resulting from replacement of a fuse is highly undesirable. A loss of oil such that a functioning fuse inside the transformer is exposed to air can cause a serious hazard to those servicing the transformer as an arc between the upper contact and the transformer wall can result.
Numerous attempts have been made to remedy the above-mentioned problem. One attempt to address this problem is by utilizing a pressure relief valve which operates automatically upon an increase in pressure in the interior of the transformer, but this valve does not entirely remedy the problem. It is common that the pressure inside the transformer is such that it falls between the pressure outside the transformer and the pressure at which the valve will automatically operate. Such a slight differential in pressure between the interior and exterior of the transformer is still sufficient to cause hot oil to be emitted from the transformer upon removal of the fuse holder. Also, some transformers do not contain a pressure relief valve.
Instead of utilizing a pressure valve that operates automatically, other transformers utilize a manually operated pressure relief valve. In such a case, the individual changing the fuse can bleed pressure from the transformer prior to removing the fuse holder. Again the problem of the emission of large quantities of oil is not entirely solved as it may be the case that the instant of time between releasing the manual pressure valve and removing the fuse holder from the housing is enough time to allow the pressure to build up enough to still emit hot oil. There is also the possibility that the operator may neglect to utilize the manually operated pressure relief valve.
Another attempt to prevent the emission of hot oil from a transformer includes the use of a spring-loaded arm and stopper mounted in the fuse housing that seals a portion of the housing when the fuse holder is removed. Such a device is disclosed in U.S. Pat. No. 5,204,654, the entire disclosure of which is incorporated by reference herein. This xe2x80x9cflapperxe2x80x9d device again does not entirely remedy the problem of the emission of large quantities of hot oil. This device operates in such a manner that the fuse holder must be removed in one quick instant to prevent the emission of large quantities of hot oil. Any moment of delay in either removing or replacing the fuse holder will allow the spring-loaded stopper to remain open and hot oil to flow from the transformer.
Other attempts to solve this problem include a hole located in the fuse holder housing that allows for the release of pressurized gas upon removal of the fuse holder. The hole is such that it extends from the interior of the fuse holder housing to the interior of the transformer tank. Upon removal of the fuse holder, pressurized gas can be released from the interior of the transformer through the hole and into the atmosphere. A potential problem exists here as it is entirely possible that the oil will have risen to such a level that it may cover the vent hole. This result will not lead to the emission of pressurized gas, but will instead result in the emission of hot oil.
The vent hole in such a device may be below the oil level in the transformer upon removal of the fuse holder for a number of reasons. As mentioned above, the oil level in the transformer will rise upon an increase in temperature inside the tank and a corresponding expansion of the oil""s volume. The vent hole may be covered as a result of pad tilt. A transformer tank is intended to be on a level surface, but it may often be the case that the transformer is improperly positioned upon installation causing the device to be tilted. It may also be the case that the ground under the tank may settle or move. Either of these two conditions present the possibility that the vent hole may be below the oil level when the operator removes the fuse holder. Thus, the potential still exists for a high velocity emission of hot oil.
Another attempt to prevent the emission of hot oil includes the use of a vent tube that can extend the vent hole described in the aforementioned device to a position higher than the location of the hole on the inside of the fuse housing. That is, the vent hole will still release pressurized gas or oil at a position within the fuse housing, but on the interior of the transformer, the vent tube will extend the positioning of the hole to a higher location.
Removal of the fuse housing in a transformer containing a vent tube as described above can still present a danger to the operator. The vent tube is still susceptible to oil entering it and emitting through the vent hole as the operator removes the fuse housing. An increase in volume of the oil or pad tilt can still cause the oil to flow into the vent tube and quickly emit through the fuse housing.
Accordingly, it is apparent that current fuse holder assemblies do not squarely solve the problem of the emission of hot oil upon removal of a bayonet-type fuse holder. Rather, each presents the possibility that a large amount of hot oil may still be lost upon fuse replacement with the further possibility of serious harm to the operator. This problem has created the need for a solution.
The fuse housing assembly of the present invention is designed to remedy the danger of the emission of hot oil or other dielectric liquid from an electrical transformer upon removal of a fuse holder from a bayonet-type fuse housing assembly. One presently preferred embodiment of the present invention achieves this goal by providing a fuse housing assembly which includes a rate release plug which allows for the emission of pressurized gas from the tank while at the same time, impeding the flow of liquid out of the tank.
Other aspects of the present invention are described below.